Emit a fullscreen toggle action on double click
If we just set "fullscreen" on the emitting video output, the fullscreen mode will not survive a change of video output. With this hack, the hotkeys interface can handle the request correctly. On the other hand, the hotkeys can also handle it incorrectly (e.g. if inside LibVLC). It all comes down to the fact that key actions are per instance, when most of them should be per-input.